1. Field of the Invention
The present invention relates to a die cushion device of a gas-filled type for use with press machines and the like.
2. Description of the Related Art
There have been provided various kinds of press machines for drawing blanks. Generally, the press machines of this type have a die and a punch to press the blank. During drawing, the working portion of a blank is drawn into the die hole by the punch, so that the peripheral portion of the blank is applied with compression strain in the circumferential direction of the blank, thus being wrinkled. For this reason, the press machines have a pressure pad for preventing wrinkles, and press a blank while holding the peripheral portion of the blank between the die and the pressure pad. When wrinkle-preventing force or load applied to the pressure pad is too weak, wrinkles are caused on the drawn piece. When load is too strong, the blank is broken by the punch while being processed. Therefore, in order to obtain a completely-drawn piece, a proper wrinkle-preventing force should be applied to the blank.
Common press machines have die cushion devices so as to generate the above-mentioned wrinkle-preventing force. There has been well known a die cushion device which generates the wrinkle-preventing force by using the spring pressure of a coil spring and the like. In the case wherein the die cushion device of this type is employed with a press machine which has a long press stroke and is used for a deep drawing, however, a spring large in dimension is needed, which makes the whole size of the device large. Further, the spring constant of the device becomes large accordingly, which naturally changes the wrinkle-preventing force to a great extent during pressing the blank, thereby making large the possibility of breaking the blank.
In order to eliminate these drawbacks of the die cushion device of the spring pressure type, there has been provided die cushion devices which utilize gas pressure. This die cushion device includes a cylinder in which gas higher than several tens barometric pressures is sealed, a piston movably inserted in the piston, and a rod extended from the piston, passing through a part of the cylinder, and connected to a pressure pad. Gas pressure in the cylinder acts on the piston to force the rod out of the cylinder. A seal is provided the slide contact portion between the rod and the cylinder or between the piston and the cylinder.
With the die cushion device of the gas pressure type, the wrinkle-preventing force can be set such a value that corresponds to the pressure of gas sealed in the cylinder at the start of pressing a blank, and the spring constant of the device can be set low by making the volume of the cylinder large. As compared with the devices using a coil spring and the like, therefore, change in the wrinkle-preventing force can be held smaller in the course of pressing a blank.
However, high pressure gas in the cylinder is sealed directly by the seal provided at the slide contact portion, so that it cannot be therefore avoided that gas leaks little by little through the seal. This causes the wrinkle-preventing force (or set load) to be reduced for a relatively short time period, thereby making it impossible to hold the predetermined wrinkle-preventing force stable. In a case wherein plural die cushions are used to push a pressure pad, the gas leakage from each of the die cushions is not uniform and this makes it impossible to apply uniform surface pressure to the pressure pad.